Floating gate electrode technology, though proposed in 1960s, is only applied in the storage field till 1980s due to the restriction caused by manufacturing technology. At present, memories such as EPROM, EEPROM and Flash widely employ the floating gate electrode technology.
The term “floating gate electrode” originated from a particular MOS transistor. This transistor has a source electrode 104, a drain electrode 105, and two gate electrode electrodes. One gate electrode has an electrical connection, and is called as a control gate electrode 100 to serve as a normal gate electrode. The other gate electrode has no external lead and is completely wrapped by two SiO2 films 102. Hence, the other gate electrode is floating and is called as a floating gate electrode 103, as shown in FIG. 1.
The operation principle of the MOS transistor with floating gate electrode is that the threshold voltage of the MOS transistor is changed depending on whether and how many charges are stored on the floating gate electrode, so as to change the external characteristics of the MOS transistor. This operation is described as follows. When a high enough voltage (such as 25V) is applied across the drain electrode and the gate electrode of the MOS transistor and the source electrode and the substrate are grounded, the PN junction between the drain electrode and the substrate is reversely broken-down and generates a large amount of high-energy electrons. These electrons pass the very thin SiOx film and accumulate on the floating gate electrode, so that the floating gate electrode is negatively charged. If the charging process continues for an enough long time, sufficient electrons will accumulate on the floating gate electrode. When the voltage applied across the drain electrode and the gate electrode is not applied any more, the electrons accumulated on the floating gate electrode will exist for a long time since there is no discharge path for these electrons accumulated on the floating gate electrode. When the floating gate electrode is negatively charged, positive charges are induced on the surface of the substrate, which increases the turn-on voltage of the MOS transistor. At this time, if the original threshold voltage that would otherwise turn on the MOS transistor is applied on the gate electrode of the MOS transistor, the MOS transistor will still be in the turn-off state. A memory cell stores binary data by using the above operation principle. The charges on the floating gate electrode generally are adjusted by the following two methods:
(1) Performing long time irradiation on the MOS transistor by ultraviolet light. While being irradiated by ultraviolet light, the electrons on the floating gate electrode form a photocurrent and hence are released.
(2) Applying high voltage between the drain electrode and the gate electrode (the drain electrode is connected with the positive terminal of the power supply, and the gate electrode is connected with the negative terminal of the power supply). This high voltage would generate a strong electric field in the SiOx film, which pulls the electrons from the floating gate electrode back into the substrate, thereby adjusting the charges on the floating gate electrode.
The floating gate electrode technology introduced above are all based on semiconductor manufacturing process, and has not appeared in the thin film transistor (TFT) manufacturing field.